Generally speaking, an optical scanning units is an optical apparatus that is used to scanning light from a light source onto an exposure object, and may typically be found in electrophotographic image forming apparatuses, such as photocopying machines, printers, and facsimiles, that reproduce images on printing media.
In operation, an electrophotographic image forming apparatus includes an optical scanning unit that irradiates the light onto and across the surface of a photosensitive drum using to form an electrostatic latent image on the drum. The electrostatic latent image is then developed using developer such as toner, and transferred and fused onto a printing medium.
A polygon mirror is typically used in an optical scanning unit of a conventional electrophotographic image forming apparatus. To realize full-color images, typically four light beams are scanned using two polygon mirrors. In order that the scanning of the light beams are synchronized with each other, the rotating speed of the spindle motors driving the two polygon mirrors needs to be synchronized with each other. In addition, when a polygon mirror is used, the surface area of the mirror needs to be sufficiently large in order to allow for the two parallel light beams to be incident on one reflective surface. The increased surface area however tends to restrict the rotating speed of the spindle mirror.
A new structure for an optical scanning unit that can substitute for, and that thus overcomes the shortcomings, such as for example, the rotating speed limitation, noise generated by the spindle motor operating at high speed and the enlarged size of the optical scanning unit, associated with, the spindle motor and the polygon mirror is thus desired.